The present invention relates to molded articles using glass as a substrate, such as a holographic optical element, an optical reflector, an encoder, etc., and a method of producing these articles. More particularly, the present invention relates to a holographic optical element and an encoder for an optical servo system used to control the track position of a read/write head of a magnetic recording disk unit, and it also relates to a method of producing the holographic optical element and the encoder.
There has heretofore been proposed a method that uses a holographic optical element and an encoder to control the track position of a read/write head of a high-density magnetic recording disk unit, as disclosed in U.S. Pat. No. 5,121,371. An outline of the proposed method will be explained below with reference to FIG. 1(a). A magnetic recording disk 10 is rotated, as shown in FIG. 1(a), and a magnetic head 15 is moved and positioned in the radial direction of the magnetic recording disk 10 under the control of a controller 19 for the purpose of recording and reading information with respect to the magnetic recording disk 10. The magnetic head 15 is integrally provided with an optical track position detecting mechanism 21 which detects the position of optical tracks circumferentially provided on the magnetic recording disk 10 at a predetermined pitch. When the magnetic recording disk 10 is provided with no optical tracks, the optical track position detecting mechanism 21 detects a position relative to an encoder 48 fixed to the magnetic recording disk unit, thereby detecting the position of the magnetic tracks on the magnetic recording disk 10. The optical track position detecting mechanism 21 includes a laser diode 27, a holographic optical element 46 having a plurality of holographic thin-line regions A to H, as shown in FIG. 1(b), a deflection mirror 47, and a photodetector 50 having four elements 51 to 54, as shown in FIG. 1(c), for detecting the optical signal reflected from the magnetic recording disk 10 or the encoder 48. In addition, the optical track position detecting mechanism 21 has a lens or a holographic optical element (not shown) for converging the optical signal reflected from the magnetic recording disk 10 or the encoder 48 on the photodetector 50.
The principle of the optical track position detecting mechanism 21 is as follows. The thin-line regions A to H of the holographic optical element 46 are paired: for example, A and B; C and D; E and F; and G and H. Each of the paired regions is provided with a hologram formed by cutting a thin-line region from one holographic lens (i.e., Fresnel zone plate-shaped imaging and collecting hologram), which is used to converge light on the surface of the magnetic recording disk 10 or the encoder 48, at a position corresponding to the region concerned. Among the rays of light emitted from the laser diode 27, convergent light beams passing through the regions A and B, for example, are reflected at the upper reflecting surface of the deflection mirror 47 and interfere with each other on the recording surface of the magnetic recording disk 10, producing interference fringes of the same pitch as that of the tracks on the magnetic recording disk 10. Accordingly, the reflected light from the surface of the magnetic recording disk 10 is intensity-modulated sinusoidally by the phase difference between the interference fringes and the tracks. Therefore, by detecting the intensity of the reflected light with the element 51, for example, of the photodetector 50, the track position can be detected. In the case of FIG. 1, convergent light beams passing through the regions C and D are also reflected at the upper reflecting surface of the deflection mirror 47 and interfere with each other on the recording surface of the magnetic recording disk 10, producing interference fringes of the same pitch as that of the tracks on the magnetic recording disk 10. However, the interference fringes are 90.degree. out of phase with respect to the interference fringes produced by the light beams passing through the regions A and B. Therefore, by using the two signals, which are 90.degree. out of phase with respect to each other, the direction of travel of the magnetic head 15 can be distinguishably detected.
On the other hand, convergent light beams passing through the paired hologram thin-line regions E and F, and G and H are reflected at the lower reflecting surface of the deflection mirror 47 and interfere with each other on the surface of the encoder 48, producing two interference fringe patterns which are of the same pitch as that of the encoder 48 but 90.degree. out of phase with respect to each other. By detecting the reflected light from the surface of the encoder 48 with the elements 53 and 54 of the photodetector 50, the track position of the magnetic recording disk 10 can be detected. This method is employed in a case where the magnetic recording disk 10 is provided with no optical tracks.
To form such a holographic optical element 46 from a relief hologram, injection molding process, pressing process, etc. have heretofore been employed as well-known producing methods. However, it is difficult to form a relief hologram as a dimple pattern having stable optical characteristics and excellent adherability and weatherability on a substrate which is smooth and readily breakable, such as a glass substrate.
Furthermore, if an aluminum film alone is provided on a substrate to form a reflector such as the conventional encoder 48, the following problems arise:
1 since aluminum is exposed, the reflector is inferior in wear resistance.
2 Since aluminum is exposed, the reflector is inferior in reflectivity.
3 Aluminum is generally inferior in adhesive power, and this tendency becomes remarkable particularly when the substrate surface is formed from a resin material.